The Tape measure tool 
on the Echogram tools toolbar is used to measure the distance between two or more points (pixels) on an active echogram. This tool provides a quick way to gauge distance on an echogram and could be useful in estimating the size of features like fish schools1, distance between artifacts, a perimeter around a object etc.
On multibeam target echograms the tool can be used to manually measure and store target length.
Distance measurements displayed on single beam echograms and multibeam echograms are described below.
Two distances are calculated and displayed on single beam echograms when the tape measure tool is used: the straight-line distance and the along-track distance.
The straight-line distance (r) between two points, A and B, on a single beam echogram is calculated as:
Where:
x1, y1, z1 are geo-referenced co-ordinates for point A representing latitude, longitude and range
x2, y2, z2 are geo-referenced co-ordinates for point B representing latitude, longitude and range
The along-track distance (s) on a single beam echogram is the sum of the distances between each ping between points A and B so that:
Where:
p is the ping number, i is the ping number of point A and n is the number of pings between A and B (inclusive)
(xp, yp, zp) are geo-referenced co-ordinates representing latitude, longitude and range for each ping
Notes:
The tape measure tool displays a distance measurement between two or more points on the echogram. The shortest distance between each two point segment is calculated with respect to the transducer relative (x,y,z) co-ordinates of each point.
The example below, shows a detected school on a multibeam echogram with a vertical Tape measure distance across the school.
The tape measure straight-line distance r is the distance between A and B and is calculated with the following equation:
Where:
x1, y1, z1 are transducer relative co-ordinates of point A
x2, y2, z2 are transducer relative co-ordinates of point B
The shortest distance between two points on a multibeam echogram may not necessarily reflect the distance due to the physical conditions.
One example is where multibeam echograms display a conical pattern of beams, as in H-mode pings. The tape measure distance will represent the path through the cone, in most cases. The exception is where two points lie on the same beam; the distance then represents the path along the surface of the cone.
Measuring the distance between two or more points
1The true dimensions of fish schools are given elsewhere in Echoview.
